ot::TimerMilliContext class

Implements a millisecond timer that also maintains a user context pointer. In typical `TimerMilli`/`TimerMicro` use, in the timer callback handler, the owner of the timer is determined using `GetOwner` method. This method works if there is a single instance of `Type` within OpenThread instance hierarchy. The `TimerMilliContext` is intended for cases where there may be multiple instances of the same class/type using a timer object. `TimerMilliContext` will store a context `void *` information.